1. Field of the Invention
The present invention relates to an optical waveguide having the function of making the diameter of propagating light smaller than that of incident light, and to a thermally-assisted magnetic recording head including the same.
2. Description of the Related Art
Recently, magnetic recording devices such as magnetic disk drives have been improved in recording density, and thin-film magnetic heads and magnetic recording media of improved performance have been demanded accordingly. Among the thin-film magnetic heads, a composite thin-film magnetic head has been used widely. The composite thin-film magnetic head has such a structure that a read head including a magnetoresistive element (hereinafter, also referred to as MR element) intended for reading and a write head including an induction-type electromagnetic transducer intended for writing are stacked on a substrate. In a magnetic disk drive, the thin-film magnetic head is mounted on a slider that flies slightly above the surface of the magnetic recording medium.
Magnetic recording media are discrete media each made of an aggregate of magnetic fine particles, each magnetic fine particle forming a single-domain structure. A single recording bit of a magnetic recording medium is composed of a plurality of magnetic fine particles. For improved recording density, it is necessary to reduce asperities at the borders between adjoining recording bits. To achieve this, the magnetic fine particles must be made smaller. However, making the magnetic fine particles smaller causes the problem that the thermal stability of magnetization of the magnetic fine particles decreases with decreasing volume of the magnetic fine particles. To solve this problem, it is effective to increase the anisotropic energy of the magnetic fine particles. However, increasing the anisotropic energy of the magnetic fine particles leads to an increase in coercivity of the magnetic recording medium, and this makes it difficult to perform data writing with existing magnetic heads.
To solve the aforementioned problems, there has been proposed a technology so-called thermally-assisted magnetic recording. The technology uses a magnetic recording medium having high coercivity. When writing data, a magnetic field and heat are simultaneously applied to the area of the magnetic recording medium where to write data, so that the area rises in temperature and drops in coercivity for data writing. Hereinafter, a magnetic head for use in thermally-assisted magnetic recording will be referred to as a thermally-assisted magnetic recording head.
In thermally-assisted magnetic recording, near-field light is typically used as a means for applying heat to the magnetic recording medium. A known method for generating near-field light is to use a plasmon generator, which is a piece of metal that generates near-field light from plasmons excited by irradiation with laser light. The laser light to be used for generating the near-field light is typically guided through an optical waveguide provided in the slider to the plasmon generator disposed near the medium facing surface of the slider.
U.S. Patent Application Publication No. 2010/0103553 A1 discloses a technology for coupling light that propagates through an optical waveguide with a plasmon generator in surface plasmon mode via a buffer part, thereby exciting surface plasmons on the plasmon generator.
When using the aforementioned technology, it is preferred that the light propagating through the optical waveguide be in single mode in the vicinity of the plasmon generator so that surface plasmons are excited on the plasmon generator with high efficiency. Meanwhile, the optical waveguide needs to have a high tolerance for misalignment of the incident light with respect to its incidence part and be able to propagate the incident light with high efficiency.
In order to increase the tolerance for the misalignment of the incident light with respect to the incidence part of the optical waveguide and promote efficient propagation of the incident light through the optical waveguide, the cross section of the optical waveguide perpendicular to the traveling direction of the light propagating through the optical waveguide may be increased in area. This, however, causes the light propagating through the optical waveguide to enter multi mode, thus precluding efficient excitation of surface plasmons on the plasmon generator.
Another approach is to use, as at least part of the optical waveguide, a spot size converter for making the spot size of the emission light smaller than that of the incident light. Such a spot size converter is disclosed in, for example, the document “Optical Technologies and Applications,” Intel Technology Journal, Vol. 8, Issue 2, pp. 153-156, May 10, 2004.
Some spot size converters for making the spot size of the emission light smaller than that of the incident light have a structure such as disclosed in the aforementioned document, where a first waveguide part having a first incidence end face and an emission end face is stacked with a wedge-shaped second waveguide part having a second incidence end face. Hereinafter, a spot size converter of such a structure will be referred to as a layered spot size converter.
In the layered spot size converter, light is incident on the first and second incidence end faces and propagates through the first and second waveguide parts. The cross section of the second waveguide part perpendicular to the traveling direction of the light decreases in area with increasing distance from the second incidence end face. This makes it difficult for the light that propagates through the second waveguide part to remain in the second waveguide part, and the light eventually moves to the first waveguide part.
In the layered spot size converter, the second waveguide part needs to be sharply pointed at its front end side (the side opposite from the second incidence end face) in order to make the light propagating through the second waveguide part move to the first waveguide part with high efficiency. For example, the front end of the second waveguide part has a radius of curvature of around 0.1 μm.
Now, problems with the layered spot size converter will be described. At the second incidence end face, the second waveguide part has a thickness and a width of, for example, several micrometers each. The second waveguide part has a length of several tens to several hundreds of micrometers, for example. In this case, the second waveguide part gradually decreases in width from several micrometers to near zero across the length of several tens to several hundreds of micrometers. This means that the second waveguide part has an extremely high thickness-to-width ratio (aspect ratio) in the vicinity of its front end in particular. The second waveguide part having such a configuration is extremely difficult to form. An example of methods for forming the second waveguide part is to form a photoresist etching mask on a dielectric layer by photolithography and pattern the dielectric layer by etching using the etching mask. In such a case, the etching mask has a portion of extremely high aspect ratio and is thus prone to collapse. Having a portion of extremely high aspect ratio, the second waveguide part itself is also vulnerable to chipping.
In the layered spot size converter, the first and second incidence end faces collectively form a single surface constituting an incidence part. Given that the spot size of the incident light is generally the same as the size of the incidence part, the tolerance for misalignment of the incident light with respect to the incidence part is not much increased in such a case.